The present invention relates to a superconducting magnet apparatus in a magnetic resonance imaging system.
A magnetic resonance imaging (MRI) system is adapted to apply a high frequency magnetic field to a test object located in a static magnetic field so as to excite, for example, hydrogen nuclei in order to measure electromagnetic waves emitted by hydrogen nuclear spins due to a nuclear magnetic resonance (which will be referred to as “NMR”) phenomenon. Then, measurement signals thereof are computed so as to create an image of a density distribution of the hydrogen nuclei in the test object which can contribute to a diagnosis for the test object. That is, in order to determine a tomography of the test object at a desired position using the MRI system, a homogenous static magnetic field in a zone to be observed is superposed thereover with an angled magnetic field for exhibiting positional data of a measurement space so as to set a predetermined magnetic field intensity in a slice cut surface having a thickness of, for example, 1 mm. Next, an electromagnetic wave having a resonance frequency is applied to the zone in order to induce an NMR phenomenon in the slice so as to generate electromagnetic waves emitted by the hydrogen nucleus ions in order to create an image.
In general, a static magnetic field in the zone to be observed in which the test object is set, requires a high static magnetic field intensity (for example, not less than 0.2 T) and a high static magnetic field uniformity (for example, about 10 ppm).
Conventionally, various electromagnet apparatuses for MRI systems have been proposed. For example, an electromagnet apparatus disclosed in U.S. Pat. No. 6,540,476 is composed of a super-conducting main coils which are opposed to each other interposing therebetween the zone to be observed. Further, in order to cancel magnetic flux induced by the pair of super-conducting main coils on the opposite sides of the zone to be observed, super-conducting shield coils having a diameter larger than that of the super-conducting coils are provided, the super-conducting shield coils being outside of and spaced from the super-conducting main coils along the axes of the super-conducting main coils. That is, the super-conducting shield coils use a current flow that is opposite in direction to the current flow in the super-conducting coils so as to cancel the magnetic flux induced on the opposite sides of the zone to be observed.
Since the intensity of the electromagnetic wave emitted by a hydrogen nucleus spin is proportional to a static magnetic field intensity, the static magnetic field intensity is increased in order to enhance the resolution of an image. In the electromagnet apparatus, in order to increase the static magnetic field intensity in a zone to be observed, both current running through the super-conducting main coils and current running through the super-conducting shield magnets is increased. As the currents running through both superconducting coils are increased, the magnetic flux passing through a space defined between both super-conducting coils is increased, and accordingly, the magnetic flux passes through the super-conducting coils themselves so as to increase the magnetic field strength, resulting in difficulty in maintaining a super-conductive state.
Accordingly, a conventional configuration was proposed wherein a disc-like ferromagnetic element is arranged between a superconducting main coil and a super-conducting shield coil so as to allow magnetic flux passing through a space defined between both super-conducting coils to concentrate to the ferromagnetic element in order to reduce the magnetic flux passing through the superconducting coils (Refer to JP-A-2001-224571, JP-A-2003-512872, JP-A-11-318858 and JP-A-11-283823).
However, since the ferromagnetic element arranged between the super-conducting main coil and the super-conducting shield coil has been conventionally disc-like, it is used in a zone having an unsaturated magnetic density, that is, it is used in an unsaturated condition. Should the ferromagnetic element be unsaturated, the magnetization characteristic curves would become uneven, thereby requiring adjustment to maintain the uniformity of the magnetic field.
For example, a technical document “R. M. Bozorth: Ferromagnetism (D. Van Nostrand. Princeton, N.J., 1951), p 849) discloses such a matter that a demagnetizing factor of about 0.01 (1/100) is obtained if the diameter D of the ferromagnetic element is about 10 times as large as the thickness T of the ferromagnetic element (that is, D/T =10). It is noted here that the demagnetizing factor of about 0.01 exhibits that the magnetization in the ferromagnetic element is decreased by 0.01 times, and accordingly, the ferromagnetic element falls in an unsaturated condition.
Explanation will be hereinbelow made of the reason why adjustment for the uniformity of the magnetic field is difficult when the ferromagnetic element falls in an unsaturated condition. The magnetization characteristic curve (BH curve) of a ferromagnetic element is as shown in FIG. 3 in which the magnetic field H is taken along the abscissa while the magnetic flux density B is taken along the ordinate. The magnetic flux density B substantially correlates to the magnetization M. As clearly understood from FIG. 3, the magnetization (magnetic flux density B) varies as the magnetic field H varies in ranges from a point “a” to a point “d” and a point “d” to a point “f,” at which the magnetization is unsaturated. Since the magnetization characteristic is uneven among materials, the adjustment for the uniformity of the magnetic field has to be made for every material to be used. A single adjustment cannot be made therefor, thus resulting in long adjustment time and the possibility that adjustment may not be possible. Meanwhile, in the case of the adjustment for the uniformity of the magnetic field in a saturated range in which the magnetic flux density is maintained to be substantially constant even through the magnetic field varies, the unevenness of the magnetization characteristic is less among materials, and accordingly, adjustment for the uniformity of the magnetic field may be facilitated.